Generally, high intensity discharge lamps such as high pressure sodium lamps and metal halide discharge lamps include an outer envelope containing an arc tube having sodium therein and a metal conductor positioned adjacent the arc tube. Also, it has long been known that arc tubes which contain sodium have a tendency to loose this sodium during the operational use of the discharge lamp.
Previous measurements have shown that sodium loss in high intensity discharge lamps results from electrolytic action induced by a surface charge of photoelectrons emitted from a metal conductor and building up on the wall surface of the arc tube. Also, it is known that this loss of sodium results in darkening of the outer envelope as well as a shortened lamp life period. Moreover, it has been determined that the rate of sodium loss is dependent upon the magnitude of the electric field across the wall of the arc tube as well as the operational temperature of the wall of the arc tube.
One prior known attempt to reduce sodium loss from the arc tube of a high intensity discharge lamp such as a metal halide lamp involves the utilization of a so-called "frameless construction". Herein, there are no side rods along the arc tube which tend to reduce photoelectron emission and electrolytic action of the sodium. However, the configuration does require a relatively long tungsten or molybdenum current return from the distal end of the arc tube to the stem associated with the outer envelope. Unfortunately, such a current return construction can lead to fractures of the stem and tube failure due to increased strain caused by the relatively long current return. Moreover, such current return material is expensive and also necessitates a relatively large outer envelope.
Also, sodium loss in high pressure sodium (HPS) lamps has been minimized by restricting the pressure of the sodium in the arc tube and by providing an excess sodium fill. However, restricting the pressure within the arc tube seriously restricts the color rendering capabilities of the lamp. Also, excess sodium fill results in lamp operation in a saturated vapor phase which is controlled by cold spot temperature and as a result the lamp is subject to voltage rise and shortened operational life.